The purification of diaryl carbonates is of great significance owing to the high requirements on the purity thereof for the preparation of qualitatively high-value polycarbonates by melt transesterification, Diaryl carbonates prepared from transesterification of an aromatic hydroxyl compound with an alkyl carbonate may comprise both high-boiling and medium-boiling secondary components, and also catalyst residues, as impurities. High-boiling components—often also referred to as high boilers—in the context of such preparation processes are those whose boiling points are above that of the diaryl carbonate. Medium-boiling components—often also referred to as intermediate boilers—in the context of such preparation processes are those whose boiling points are between that of the diaryl carbonate and that of the alkyl aryl carbonate which can be formed as a by-product during the preparation of the diaryl carbonate. All of these impurities can lead to considerable qualitative impairment of the polycarbonates to be prepared and have to be removed by appropriate purification before the further use of the diaryl carbonates.
International Patent Publication No. WO 2004/113264, the entire contents of which are hereby incorporated herein by reference, describes a process for recovering a product stream from a waste stream in the preparation of diaryl carbonate. Also described is a refining of the diaryl carbonate which has been prepared in a reaction using three reaction columns, and which is carried out in three stages including catalyst removal, low boiler removal and high boiler removal. Both in the first and in the latter step, the diaryl carbonate is removed as the top product. The process described is not only exceptionally complex in apparatus terms, but also energetically unfavorable owing to the removal of the diaryl carbonate both in the first stage and in the third stage as the top product. In addition, the quality of the diaryl carbonate thus prepared is very poor at 99.5% by weight and is unsuitable for the preparation of qualitatively high-value polycarbonate.
European Patent Publication No. EP 1 760 069 A, the entire contents of which are hereby incorporated herein by reference, describes a process for preparing diaryl carbonates, in which, after the transesterification of an aromatic hydroxyl compound with an alkyl carbonate, a mixture is obtained which, as well as diaryl carbonate, comprises an aromatic carbonate ether as an impurity among other impurities. The latter is removed in a subsequent step in order to obtain a high-purity diaryl carbonate. The significantly more complex removal of high-boiling secondary components, which would lead to qualitative impairment of the target products polycarbonates), is, however, not described.
European Patent Publication Nos. EP 1 783 112 A1, EP 1 787 977 A1, EP 1 801 095 A1 and EP 1 787 976 A1, the entire contents of each of which are hereby incorporated herein by reference, describe a process consisting of at least two stages for purifying diaryl carbonate, especially diphenyl carbonate, in which the high-boiling components and the catalyst are first removed after the reaction. The distillate obtained in this high boiler removal is separated into three fractions in a second distillation column, diaryl carbonate being withdrawn in the sidestream. However, a disadvantage of this process is that the high-boiling components are removed in the first step, since this is energetically unfavorable.
European Patent Publication No. EP 784 048 A, the entire contents of which are hereby incorporated herein by reference, describes a process for purifying diaryl carbonates, in which a distillation is carried out at a bottom temperature greater than 150° C. and the product withdrawal of the purified diaryl carbonate is effected in the sidestream of the column. The diaryl carbonates to be purified can be prepared either by reaction of carbonyl halides with aromatic hydroxyl compounds or by transesterification of aromatic hydroxyl compounds with dimethyl carbonate. Although the process described enables diaryl carbonates to be freed very substantially of high boilers, for example phenyl salicylate, nowhere is the presence of troublesome catalyst residues or intermediate boilers or a means for their removal described. Especially the presence of significant amounts of catalyst residues can, however, influence a distillative purification, since this accelerates the side reaction of diaryl carbonate to give phenyl salicylate and hence promotes the enrichment thereof in the bottom of the column. Therefore, the process described in EP 784 048 A is not immediately suitable for such a purification in the presence of catalyst residues and/or intermediate boilers.